It's because the injected vaccine is much, much more expensive. The vaccine itself is about 14x the price of the oral version, then you also need the injection equipment and a trained person to administer it, all of which are non-trivial costs in this context - they've administered something like ten billion doses of vaccine.

The plan is that when the wild virus is eradicated (Africa has just been declared to have eradicated the virus in the wild) then they'll switch to the inactivated vaccine For continuing management. They've already switched from the trivalent formula to the bivalent formula after eliminating WPV2, which is a substantial decrease in the chance of vaccine-acquired infection.

Bitch about it all you want, but polio has been reduced by like 99% over the past 25 years because of immunization programs. Sometimes actually achieving outcomes in a practical way is more important than trying to be flawless and failing entirely.